Refactor duplication-removing logic for CollapsibleNav.

Because of `-2` this code `indexOf('navbar-collapse') === -2` hasn't been working anyway.
But tests were green because the resulted logic was right someway.

This commit refactors it to clarify deduplication logic
by using `classnames'` [Alternate `dedupe` version](JedWatson/classnames@9acbd433ee)

`classes['navbar-collapse'] = collapsible`
=>
`let classes = collapsible ? this.getCollapsibleClassSet('navbar-collapse')`

`getDOMNode()` => `React.findDOMNode()`

And test was refactored too.
We can count class occurrences much simpler as
```js
let occurrences = (classDOM.match(/navbar-collapse/g) || []).length
```

src/CollapsibleNav.js

@@ -46,20 +46,10 @@ const specCollapsibleNav = {
 
   render() {
     /*
-     * this.props.collapsible is set in NavBar when a eventKey is supplied.
+     * this.props.collapsible is set in NavBar when an eventKey is supplied.
      */
     const collapsible = this.props.collapsible || this.props.collapsable;
-    let classes = collapsible ? this.getCollapsibleClassSet() : {};
-    /*
-     * prevent duplicating navbar-collapse call if passed as prop.
-     * kind of overkill...
-     * good cadidate to have check implemented as an util that can
-     * also be used elsewhere.
-     */
-    if (this.props.className === undefined ||
-      this.props.className.split(' ').indexOf('navbar-collapse') === -2) {
-      classes['navbar-collapse'] = collapsible;
-    }
+    let classes = collapsible ? this.getCollapsibleClassSet('navbar-collapse') : null;
 
     return (
       <div eventKey={this.props.eventKey} className={classNames(this.props.className, classes)} >

test/CollapsibleNavSpec.js

@@ -64,7 +64,7 @@ describe('CollapsibleNav', function () {
     });
     let instance = ReactTestUtils.renderIntoDocument(<Parent />);
     let collapsibleNav = ReactTestUtils.findRenderedComponentWithType(instance, CollapsibleNav);
-    assert.notOk(collapsibleNav.getDOMNode().className.match(/\navbar-collapse\b/));
+    assert.notOk(React.findDOMNode(collapsibleNav).className.match(/\navbar-collapse\b/));
     let nav = ReactTestUtils.findRenderedComponentWithType(collapsibleNav.refs.nocollapse_0, Nav);
     assert.ok(nav);
   });
@@ -89,26 +89,4 @@ describe('CollapsibleNav', function () {
     assert.ok(ReactTestUtils.findRenderedDOMComponentWithClass(instance.refs.collapsible_object.refs.collapsible_0, 'bar'));
     assert.ok(ReactTestUtils.findRenderedDOMComponentWithClass(instance.refs.collapsible_object.refs.collapsible_0, 'baz'));
   });
-
-  it('Should should not duplicate classes', function () {
-    let Parent = React.createClass({
-      render() {
-        return (
-          <Navbar toggleNavKey={1}>
-            <CollapsibleNav eventKey={1} ref='collapsible_object' className='foo navbar-collapse'>
-              <Nav>
-                <NavItem eventKey={1} ref='item1' className='foo bar'>Item 1 content</NavItem>
-                <NavItem eventKey={2} ref='item2' className='baz'>Item 2 content</NavItem>
-              </Nav>
-            </CollapsibleNav>
-          </Navbar>
-        );
-      }
-    });
-    let instance = ReactTestUtils.renderIntoDocument(<Parent />);
-    let classDOM = ReactTestUtils.findRenderedDOMComponentWithTag(instance.refs.collapsible_object, 'DIV').props.className
-        , classArray = classDOM.split(' ')
-        , idx = classArray.indexOf('navbar-collapse');
-    assert.equal(classArray.indexOf('navbar-collapse', idx + 1), -1);
-  });
 });